The present invention relates to apparatus for optically reading information such as bar code through a hologram device.
One known optical information reading technique (bar code reader or image scanner) involves optically reading information through a reflection type hologram device or a transmission type hologram device such as described in the Japanese Patent Provisional Publication No. 1-142523. FIG. 1 is an illustration of a conventional bar code reader using a reflection type hologram device. In FIG. 1, a light beam emitted from a light source 11 is condensed through a lens 14 to produce a condensed light beam 11a which in turn, passes through a hole of a mirror 14 to reach a reflection type hologram device 13 comprising a plurality of hologram elements 13a, then reflected and diffracted thereon so as to produce a reflected and diffracted light beam 11b to be directed to a bar code 17. At this time, a motor 12 is driven so as to rotate the reflection type hologram device 13, whereby the bar code 17 is scanned with the reflected and diffracted light beam 11b. On the other hand, a reflected light beam 17a from the bar code 17 is incident on the hologram element 13 a, thereafter again reflected, diffracted and condensed thereat to be directed to the mirror 15. The light reflected on the mirror 15 is led to a light-receiving device 16. The light-receiving device 16 photoelectrically converts the received light into an electric signal which is led to a decoder circuit, not shown, to decode the optical information corresponding to the bar code 17.
There is a problem which arises with such an optical information reading apparatus, however, in that for the reception of the reflected light from the bar code 17 the light-reception surface of the hologram device 13 is required to be relatively wide. That is, the reflected light from the bar code 17 is scattered light and is spread out when led to the hologram device 13. Thus, for obtaining the minimum optical intensity necessary to accurately read out the bar code information, the hologram device 13 is required to have a relatively wide light-reception area. That is, this causes enlargement of the diameter of the hologram device 13 because the hologram device 13 is composed of a plurality of hologram elements 13a. The excessive enlargement of the diameter of the hologram device 13 increases the inertial mass of the hologram device 13 in response to rotation itself, thereby deteriorating the rotation rising characteristic and the anti-vibration characteristic. As a result, there is the possibility that difficulty is encountered in ensuring the accurate communication of the reflected light from the bar code to the light-receiving device 16 and hence to accurately read out the bar code information. In addition, for the aforementioned problems, one possible solution is to enlarge the diameter of the motor 12. However, this solution causes size-enlargement of the motor 12 and hence the enlargement of the apparatus so as to raise the manufacturing cost of the apparatus.